Molecular Mechanism of Uptake of Cationic Photoantimicrobial Phthalocyanine across Bacterial Membranes Revealed by Molecular Dynamics Simulations

Phthalocyanines
are aromatic macrocyclic compounds, which are structurally
related to porphyrins. In clinical practice, phthalocyanines are used
in fluorescence imaging and photodynamic therapy of cancer and noncancer
lesions. Certain forms of the substituted polycationic metallophthalocyanines
have been previously shown to be active in photodynamic inactivation
of both Gram-negative and Gram-positive bacteria; one of them is zinc
octakis­(cholinyl)­phthalocyanine (ZnPcChol8+). However,
the molecular details of how these compounds translocate across bacterial
membranes still remain unclear. In the present work, we have developed
a coarse-grained (CG) molecular model of ZnPcChol8+ within
the framework of the popular MARTINI CG force field. The obtained
model was used to probe the solvation behavior of phthalocyanine molecules,
which agreed with experimental results. Subsequently, it was used
to investigate the molecular details of interactions between phthalocyanines
and membranes of various compositions. The results demonstrate that
ZnPcChol8+ has high affinity to both the inner and the
outer model membranes of Gram-negative bacteria, although this species
does not show noticeable affinity to the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine membrane. Furthermore,
we found out that the process of ZnPcChol8+ penetration
toward the center of the outer bacterial membrane is energetically
favorable and leads to its overall disturbance and formation of the
aqueous pore. Such intramembrane localization of ZnPcChol8+ suggests their twofold cytotoxic effect on bacterial cells: (1)
via induction of lipid peroxidation by enhanced production of reactive
oxygen species (i.e., photodynamic toxicity); (2) via rendering the
bacterial membrane more permeable for additional Pc molecules as well
as other compounds. We also found that the kinetics of penetration
depends on the presence of phospholipid defects in the lipopolysaccharide
leaflet of the outer membrane and the type of counterions, which stabilize
it. Thus, the results of our simulations provide a detailed molecular
view of ZnPcChol8+ “self-promoted uptake”,
the pathway previously proposed for some small molecules crossing
the outer bacterial membrane.